Mark 9:28

And when he was come into the house, his disciples asked him privately, How is it that we could not cast it out? And he said unto them, This kind can come out by nothing, save by prayer.

Asked him privately ... It was well for the reputation of the apostles that they sought a private answer, for they were grievously at fault. Matthew quoted Jesus as saying their failure was due to their "little faith" (Matthew 17:20), and Mark's words indicate either a failure to pray at all or some serious lack in their prayers. Even the greatest miracles performed by Jesus were done so in answer to prayer (John 9:31; John 11:41); and, although the mention of the Saviour's prayers in connection with his mighty deeds was not always included by the sacred writers, the assumption must ever be that all of them included the Saviour's asking of God in prayer the accomplishment of the wonders recorded. The failure of the apostles here seems to have been that of omitting prayer. That they fully expected to succeed is evident, so their faith was not that of failing to expect success, but of taking it for granted that they could succeed without praying to God for the expected blessing. The apostles had often succeeded before (Mark 6:13,30); and they perhaps believed that they had the power IN THEMSELVES to continue doing such things.

They had to learn that God's power is not given to men in that way. It has rather ever to be asked for afresh. To trust in God's power in the sense that we imagine we have it in our control and at our disposal ... is to trust in ourselves instead of in God.[17]